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Thread: NiMH recharging- how hot is too hot?

  1. #1
    Unenlightened LuxChaser's Avatar
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    Thinking NiMH recharging- how hot is too hot?

    I'm trusting that everyone is enjoying a well illuminated and safe holiday
    and will continue doing so!

    my duracell 2650, eneloops, energizer 2500 and hybrids(all AA) get
    stupendously hot when charging the final 1/3 of the cycle. As in can't
    keep them in my grasp (160+ degrees?). Is this a normal? I'm using a
    DX Soshine "smart" charger which supposedly has thermal protection
    but apparently does not kick in. Is it time to upgrade my charger?
    I'm not going to ask which charger as I can use the search function,
    but my question is for anyone choosing to answer, how hot is hot?

  2. #2
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    Default Re: NiMH recharging- how hot is too hot?

    Hello LuxChaser,

    Welcome to CPF.

    Heat destroys NiMh cells. 160 F is too hot.

    Ideally your cells may see a 10 - 20 F rise over ambient at the end of the charge. The best performance comes when the cells are in the 120 - 140 F range, but the higher the temperature the more it cuts into cycle life.

    There are basically two reasons for cells heating up. Well maybe three...

    Too high a charge rate. This is a rate greater than 1C, or a charge that completes in under an hour.

    High internal resistance of the cell. As cells age, their internal resistance increases.

    The charger misses the end of charge signal. This is usually caused by improper charge rates, but some chargers are better than others. The battery manufacturers recommend charging in the 0.5 - 1.0C rate, or charging at 0.1C for 16 hours.

    Tom
    Behind every Great man there's always a woman rolling her eyes...

    Most batteries don't die - they are tortured to near death, then murdered...

  3. #3
    Flashaholic gravityz's Avatar
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    Default Re: NiMH recharging- how hot is too hot?

    i have had the same experience

    i bought a charger incl 4 cells

    the charger was a low budget onw which charges at about 700ma

    somehow the cells got real hot and eventually destroyed them

    now i have a new charger with display which can select the carge(200, 500, 700, 1000)

    even with this charger i notice that when i select 700ma on a 2500mah battery the battery gets a bit hot when the chargecycle is nearly completed.
    it is not that hot that i can not touch it but i guess somewhere around 60 degrees centigrade.
    maybee it is best to charge with 1000 for lest say 1.5 hours than start a new cycle with 500ma

    anyway the thermal protection is only there to avoid fire hazards. i do not think the charger monitors the cell to avoid damaging

  4. #4
    Flashaholic*
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    Default Re: NiMH recharging- how hot is too hot?

    On a 2500mAh cell, you should try to charge at .5-!C or 1/2-Full of the cell's capacity as there's a chance of missing termination which causes lots of heat!

    You should charge those cells at 1300-2500ma but since you can only charge at 1000ma or the highest, try that and monitor the heat! If that doesn't help, there's something wrong with your charger.
    Bill

    I'm a retired mechanic not a electronic/electrical engineer!

  5. #5
    Flashaholic gravityz's Avatar
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    Default Re: NiMH recharging- how hot is too hot?

    Quote Originally Posted by wptski View Post
    On a 2500mAh cell, you should try to charge at .5-!C or 1/2-Full of the cell's capacity as there's a chance of missing termination which causes lots of heat!

    You should charge those cells at 1300-2500ma but since you can only charge at 1000ma or the highest, try that and monitor the heat! If that doesn't help, there's something wrong with your charger.

    so you mean that charging these high caspacity cells with a lower charge can actually get them to heat up more.

    i am currently doing some tests with the use of a laser thermometer.

    i charged 2x 2500mah batteries who were almost full already
    charged them with 700ma
    temp felt warm but actually is 37 degreess

    what is the savest temp before cells start to get damaged?

  6. #6
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    Default Re: NiMH recharging- how hot is too hot?

    Hello Gravityz,

    If you can keep your cells in the 30 - 40 C range you should be OK.

    We know damage occurs at 60 C. Below that its a bit fuzzy as to the exact temperature where damage occurs.

    Tom
    Behind every Great man there's always a woman rolling her eyes...

    Most batteries don't die - they are tortured to near death, then murdered...

  7. #7
    Flashaholic gravityz's Avatar
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    Default Re: NiMH recharging- how hot is too hot?

    ok

    i will do some tests(since nobody else apparently did)

    i will discharge 4 2500mah AA cells to 1 volt and then charge them with

    1500 ma, 1000 and 700 (in that order)

    i just discovered that when i put 2 cells in instead of 4 i can go up to 1800ma charge.

    sanyo shows specs that charge a cell with 2000ma but i think that is a bit risky without cooling.

    will put the results in this thread.

  8. #8
    Flashaholic*
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    Default Re: NiMH recharging- how hot is too hot?

    Quote Originally Posted by gravityz View Post
    so you mean that charging these high caspacity cells with a lower charge can actually get them to heat up more.

    i am currently doing some tests with the use of a laser thermometer.

    i charged 2x 2500mah batteries who were almost full already
    charged them with 700ma
    temp felt warm but actually is 37 degreess

    what is the savest temp before cells start to get damaged?
    Normal charging sends the cell into a wee bit of overcharge, so if it misses termination, heat climbs fast. Cells "can" miss termination at lower charge rates more than high ones.
    Bill

    I'm a retired mechanic not a electronic/electrical engineer!

  9. #9
    Flashaholic* 65535's Avatar
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    Default Re: NiMH recharging- how hot is too hot?

    On hobby grade chargers, you can set the peak detection voltage drop from about 3mV-15mV the higher higher the peak detection voltage the higher the chances of missed termination, on older cells that can false peak (peak before they are actually fully charged) you increase the peak detection.

    Since the cells end up with higher internal resistance after every cycle, they will get hotter every charge, it's very minimal but it's there. As long as you charge at around 1C or less your cells will last plenty long, if you want the most out of your cells you'll need to charge them at low rates and with a low peak detection voltage.

    All battery chemical reactions are endothermic, charge a cell it gets hot discharge a cell it gets hot.
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  10. #10

    Default Re: NiMH recharging- how hot is too hot?

    I had a bad experience with a smart deltaV charger, I tried to charge batteries that were almost fully charged, the charger did not find the deltaV drop and continued charging at 1C and the batteries got to >60C and leaked.

    since then I carefully monitor my charging with deltaV chargers.

    As a better solution i built a deltaT charger based on Maxim 2715 chip available from digikey. The NTC temperature sensor is available from rapidelec.co.uk

    The Maxim 2715 will not start charging if the cell temperator is >45C and will stop charging is the cell temp >50C It uses a deltaT of 0.5C/min to stop the initial change (I use a 1C initial charge) and then charges at 1/3C for 30mins top up. If the cell voltage is <1.0V per cell it provides a pre-charge to help recover over discharged cells.

    I have done a number of charge/discharge tests and the batteries seem to get a full charge. Also charging fully charged batteries safely terminates the 1C charge is a few minutes on deltaT.

    The only thing I have found is that wrapping the cells in bubble wrap when charging result in under charge. This implies the temperature sensor should be in close contact with the cells but that the cells themselves should have free air to dissipate the heat.

    All testing has been at 25C. I have not tried high ambient charging at say 35C to 40C (yesterdays temp in Melbourne was 40C but I am in Sydney :-) )

    You can run the DS2715 either as a switcher or a linear buck regulator. I am using it in linear buch regulator mode as the board layout is less critical.

    p.s. the data sheet for the Eneloop AA (which I use) gives fast charge as 1.1Hr at 1C and a charging ambient temp range of 0C to 40C, discharge ambient temp 0C to 50C. Note this is ambient the cells themselves will be (can be) hotter the 40C when charging and hotter then 50C when discharging.
    Last edited by mpf; 12-31-2007 at 06:25 PM.

  11. #11
    Flashaholic gravityz's Avatar
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    Default Re: NiMH recharging- how hot is too hot?

    ok did tests with various charging levels
    i tested 4 batteries, temperature are the min and max ones from every run.
    battery:NiMH 2500MaH

    charging current:1500ma
    temp at 1-1.4 volts=39 degrees
    temp at 1.46-full volts=44-46 degrees

    charging current:1000ma
    temp at 1-1.4 volts=34 degrees
    temp at 1.46-full volts=34-38 degrees

    charging current:700ma
    temp at 1-1.4 volts=34 degrees
    temp at 1.46-full volts=43-47 degrees

    so this indeed means that NiMH should be charged with about .5C to get the lowest temp.

    a low charge does not mean temp will be lower also because it probably keeps on charging slowly

  12. #12
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    Default Re: NiMH recharging- how hot is too hot?

    Hello Gravityz,

    The actual trend continues down until you get above 1C. The reason you are seeing higher temperatures at 1500 mA than at 1000 mA is because the charger is heating up, and thus heating the cells up. If you were able to charge the cells separate from the charger, you should see 1500 mA charging have slightly lower temperatures than 1000 mA charging.

    Please note that this is for "healthy" cells.

    I would speculate that the reason your cells heated up during the 700 mA charge was because of a missed end of charge termination signal. The reason your cells heated up during the 1500 mA charge is because the charger heated up and passed some of its heat to the cells.

    Tom
    Behind every Great man there's always a woman rolling her eyes...

    Most batteries don't die - they are tortured to near death, then murdered...

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